Module 6 · Process Improvement with BPMN and Lean
Bottlenecks and flow
Find the constraint that limits a process, use Little's law to connect work in progress, throughput and lead time, and see why improving anything except the bottleneck changes little.
About 20 minutes
The problem
Harbourline's operations director has a budget for one improvement this quarter. The documentation team wants a faster scanner. The customs brokers want a second laptop. The haulage team wants another truck. All three would make their own step quicker. Which would get containers out of the port sooner?
Probably none of them. A process can only move as fast as its slowest point, its bottleneck or constraint, and time saved anywhere else just means items reach the bottleneck sooner and wait there instead. Before spending money on speed, find the constraint.
The concept
The theory of constraints in five steps
- Identify the constraint: where does work pile up? Where are the longest waits?
- Exploit it: make sure the constraint never wastes time (no idle inspection slots, no files arriving incomplete).
- Subordinate everything else to it: schedule the other steps to feed it smoothly, not to look busy.
- Elevate it: if it's still the limit, add capacity there (more inspection slots, earlier booking).
- Repeat: once it's no longer the constraint, something else will be. Find it.
Signs of a bottleneck in data
- The longest average wait sits just before it.
- Waits before it get longer when more work arrives at once.
- Cases that skip it finish much faster.
Little's law
For a stable process, over time:
Work in progress (WIP) = throughput × lead time
If 2.8 clearances arrive a day and each spends 8.6 days at port, about 24 clearances are at the port at any moment. Turn it around: to cut lead time without cutting throughput, you must cut the work in progress, which means removing waiting, not hurrying the work.
Example
Harbourline's clearances, before the pilot, by customs channel:
| Channel | What happens | Days at port |
|---|---|---|
| Green | no inspection | 7.6 |
| Yellow | document review by customs | 8.7 |
| Red | physical inspection | 9.3 |
Red cases spend about 1.7 days longer at port than Green ones, and the wait before physical inspection (42 hours on average) grows when several Red containers arrive in the same few days. The customs inspection queue behaves like a bottleneck for the 4 in 10 containers that go through it. But look back at the value stream: the waits for corrected documents and duty payment are as long or longer, and they affect every channel. There's more than one constraint, and some of them sit with Harbourline's own customers.
Walkthrough
- Calculate days at port by customs channel, before the pilot.
- Calculate the arrival rate (clearances per day) for January to April, and apply Little's law (the first task below).
- Look at the wait before physical inspection over time. Is it longer in weeks with more Red arrivals?
- Rank the constraints: inspection queue (controlled by customs), corrected documents and duty payment (controlled by customers, but influenced by Harbourline).
- For each, ask what "exploit" would mean: for example, having every Red file complete and paid before its inspection slot comes up.
Practice
Practice
Before the pilot, how many more days at port did Red channel clearances spend than Green ones, on average? Two decimal places.
Practice
Using Little's law, about how many clearances were at the port at any one time before the pilot? Use 2.82 arrivals a day and the pre-pilot average of 8.63 days at port. One decimal place.
Task
8 minThe operations director can fund one of: a faster scanner for the documentation team, a second laptop for the customs brokers, or another truck for haulage. Write a short recommendation (60 to 150 words) explaining why none of them would shorten days at port much, and what you'd do instead, using the idea of a constraint and at least two numbers from the data.
Your work is checked for
- Uses the idea of a bottleneck or constraint
- At least two numbers
- Mentions waiting
- Proposes something aimed at documents, payment or inspection
- Between 60 and 150 words
Check your understanding
Answer every question to check.